stbt power: Tool to control networked power switch

Based on a script written by Lewis Haley; I added error checking
and other minor fixes.

This currently supports 2 types of network-controllable power devices:

* IP Power 9258, a family of devices sold under various brand names,
  e.g. http://www.aviosys.com/9258st.html
  (in our experience these devices consistently fail within a year of
  use).

* PDUeX KWX line:
  http://www.pdu-expert.eu/index.php/en/component/k2/itemlist/category/1

I haven't personally tested this against the "IP Power 9258" device
because I don't have the hardware to test against.
stbt power: Check and print new status after switching on/off

The exit status (and the standard output) now reflect whether the
outlet's new status is the same as the status you requested. Before, the
command might fail with no indication that it had failed.

The sleep is necessary because the previous command returns before the
change actually takes effect.